Composite of metal oxide nanoparticles and carbon, method of production thereof, electrode and electrochemical element employing said composite

ABSTRACT

A composite powder in which highly dispersed metal oxide nanoparticle precursors are supported on carbon is rapidly heated under nitrogen atmosphere, crystallization of metal oxide is allowed to progress, and highly dispersed metal oxide nanoparticles are supported by carbon. The metal oxide nanoparticle precursors and carbon nanoparticles supporting said precursors are prepared by a mechanochemical reaction that applies sheer stress and centrifugal force to a reactant in a rotating reactor. The rapid heating treatment in said nitrogen atmosphere is desirably heating to 400° C. to 1000° C. By further crushing the heated composite, its aggregation is eliminated and the dispersity of metal oxide nanoparticles is made more uniform. Examples of a metal oxide that can be used are manganese oxide, lithium iron phosphate, and lithium titanate. Carbons that can be used are carbon nanofiber and Ketjen Black.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. patent applicationSer. No. 13/638,542 filed on Sep. 28, 2012.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a composite of metal oxidenanoparticles and carbon, a method of production thereof, an electrodeand an electrochemical element that employ this composite.

2. Description of Related Art

A carbon material etc. that stores and releases lithium is currentlyused as the electrode for lithium batteries, but there is a risk ofdecomposition of the electrolytic solution since the negative potentialis lower than the reductive decomposition potential of hydrogen.Accordingly, as described in Patent Documents 1 or 2, the use of lithiumtitanate having higher negative potential than the reductivedecomposition potential of hydrogen is being investigated, but lithiumtitanate has a problem of low output property. Accordingly, attempts arebeing made to improve output property by an electrode in which lithiumtitanate is made into nanoparticles and supported on carbon.

The inventions described in these Patent Documents are methods forapplying sheer stress and centrifugal force to a reactant in a rotatingreactor to allow promotion of chemical reaction (generally referred toas mechanochemical reaction), thereby yielding dispersed lithiumtitanate supported on carbon. In this case, for example, titaniumalkoxide and lithium acetate which are the starting materials of lithiumtitanate, a carbon such as carbon nanotube or Ketjen Black, and aceticacid etc. are used as reactants.

Although the electrodes that use carbon supporting lithium titanatenanoparticles described in these Patent Documents exert superior outputproperty, there are recent demands to further improve the outputproperty and to improve the electric conductivity in this type ofelectrode.

There are also demands to produce a composite in which not only lithiumtitanate nanoparticles but other metal oxide nanoparticles are supportedon carbon, and thereby obtain an electrode or an electrochemical elementhaving a more superior output property. In particular, the use of ametal oxide cheaper than lithium such as manganese oxide is desired.

The present invention is proposed to solve the problems of theconventional technology as stated above, the object of which is toprovide a composite of metal oxide nanoparticles and carbon that canyield an electrode or an electrochemical element that enablesimprovement of output property and electric conductivity, as well as amethod of production thereof. In addition, another object of the presentinvention is to provide an electrode and an electrochemical element thatemploy said composite.

SUMMARY OF THE INVENTION

In order to achieve the said object, in a method for producing acomposite of metal oxide nanoparticles and carbon according to thepresent invention, sheer stress and centrifugal force is applied to asolution including a starting material of metal oxide and carbon powderin a rotating reactor to allow reaction and obtain a composite powder inwhich highly dispersed metal oxide nanoparticle precursors are supportedon carbon under nitrogen atmosphere, and the composite powder is rapidlyheated under nitrogen atmosphere to allow progression of crystallizationof metal oxide so that metal oxide nanoparticles having ultra thin filmstructure is highly dispersed and supported by the carbon. In this case,it is also aspects of the present invention that said rapid heatingtreatment is heating the composite under nitrogen atmosphere to400-1000° C., and that the metal oxide nanoparticles have a thickness of1 nm or less at 2-5 atomic layers level and is a crystal structure(ultra thin film structure) on a plate of 5-100 nm in diameter. Further,a composite produced with a method as above, as well as an electrode oran electrochemical element that this composite are encompassed in thepresent invention.

According to the present invention, good crystallization of metal oxidenanoparticles can be progressed by rapid heating treatment in thecalcination step of carbon supporting the metal oxide nanoparticleprecursors, and a crystal structure having a thickness of 1 nm or lessat 2-5 atomic layers level on a plate of 5-100 nm in diameter is formed.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the present invention, which are believed tobe novel, are set forth with particularity in the appended claims. Thepresent invention, both as to its organization and manner of operation,together with further objects and advantages, may best be understood byreference to the following description, taken in connection with theaccompanying drawings.

FIG. 1 is the result of XRD analysis and substitute for drawing whichshows the TEM image of the composite of Example 1;

FIG. 2 is a photographic substitute for drawing which shows themagnified TEM image of other portions of the composite of Example 1;

FIG. 3 is a photograph as substitute for drawing which shows themagnified TEM image of other portions of the composite Figurephotographic of Example 1;

FIG. 4 is the result of XRD analysis and a substitute for drawing whichshows the TEM image of the composite of Example 2-1;

FIG. 5 is a photograph as substitute for drawing which shows the TEMimage of other portions of the composite of Example 2-1;

FIG. 6 is a photograph as substitute for drawing which shows themagnified TEM image of other portions of the composite of Example 2-1;

FIG. 7 is a photograph as substitute for drawing 15 which shows the highresolution TEM image of Example 2-1;

FIG. 8 is a graph showing the charge and discharge property of theelectrochemical element that uses the composite of Example 2-1;

FIG. 9 is a graph comparing the charge and discharge property of theelectrochemical element that uses the composite of Example 2-1 with thatof a conventional electrochemical element;

FIG. 10 is a graph showing the output property of the electrochemicalelement that uses the composite of Example 2-1;

FIG. 11 is a graph showing the cycle property of the electrochemicalelement that uses the composite of Example 2-1;

FIG. 12 is a photograph as substitute for drawing which shows the highresolution TEM image of the composite of Example 2-2;

FIG. 13 is a photograph as substitute for drawing which shows the highresolution TEM image of the composite of Example 2-3;

FIG. 14 is a photograph as substitute for drawing which shows the highresolution TEM image of the composite of Example 2-5;

FIG. 15 is a photograph as substitute for drawing which shows the TEMimage of the composite of lithium titanate nanoparticles and carbon ofExample 3;

FIG. 16 is a photograph as substitute for drawing which shows themagnified TEM image of the composite of lithium titanate nanoparticlesand carbon of Example 3;

FIG. 17 is graphs showing the discharge behavior property of thecomposite of lithium titanate nanoparticles and carbon of Example 3;

FIG. 18 is graphs showing the output property of the composite oflithium titanate nanoparticles and carbon of Example 3;

FIG. 19 is a graph showing the property of the high output energystorage device of Example 4;

FIG. 20 is a perspective view showing an example of the reactor used inthe production method of the present invention;

FIG. 21 is a photograph as substitute for drawing and graphs shows themicropore distribution of the composite of lithium iron phosphatenanoparticles and carbon of Example 2-1;

FIG. 22 is a graph showing the charge and discharge property of Example5;

FIG. 23 is a graph showing the result of XRD analysis of Example 5;

FIG. 24 is a magnified photograph of Example 2-1 and a schematic diagramthereof; and

FIG. 25 is a high resolution TEM image of the composite powder ofExample 2-3 and a schematic diagram thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of theinvention which set forth the best modes contemplated to carry out theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction with thepreferred embodiments, it will be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents, which may be included within the spirit and scope of theinvention as defined by the appended claims. Furthermore, in thefollowing detailed description of the present invention, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it will be obvious toone of ordinary skill in the art that the present invention may bepracticed without these specific details. In other instances, well knownmethods, procedures, components, and circuits have not been described indetail as not to unnecessarily obscure aspects of the present invention.

(Mechanochemical Reaction)

The reaction method employed in invention is a mechanochemical reactionthe similar present to the method shown in Patent Documents 1 and 2previously filed for patent application by the present inventors,wherein sheer stress and centrifugal force are applied to a reactant ina rotating reactor during the chemical reaction process to allowpromotion of chemical reaction.

This reaction method can be performed employing for example the reactoras shown in FIG. 20. As shown in FIG. 20, the reactor consists of anouter tube 1 having a sheathing board 1-2 at the opening and a rotatinginner tube 2 having through-holes 2-1. By introducing the reactant intothe inner tube of this reactor and rotating the inner tube, the reactantinside the inner tube is transferred through the through-holes of theinner tube to the inner wall 1-3 of the outer tube by its centrifugalforce. At this time, the reactant collides with the inner wall of theouter tube by the centrifugal force of the inner tube, and slides up tothe upper portion of the inner wall in a thin film state. In this state,the sheer stress with the inner wall and the centrifugal force from theinner tube are both simultaneously applied to the reactant, mechanicalenergy is thereby applied to reactant. This mechanical energy is and alarge the thin film thought to be converted into chemical energynecessary for reaction, the so-called activation energy, and thereaction is progressed in a short period of time.

In this reaction, since the mechanical energy applied to the reactantwill be large when in a thin film state, the thickness of the thin filmis 5 mm or less, preferably 2.5 mm or less, and further preferably 1.0mm or less. The thickness of the thin film can be set by the width ofthe sheathing board and the amount of the reaction solution.

It is thought that this reaction method can be realized by themechanical energy of sheer stress and centrifugal force applied to thereactant, and this sheer stress and centrifugal force are generated bythe centrifugal force applied to the reactant inside the inner tube.Accordingly, the centrifugal force applied to the reactant inside theinner tube necessary for the present invention is 1500 N (kgms⁻²) orhigher, preferably 60000 N (kgms⁻²) or higher, and further preferably270000 N (kgms²) or higher.

In this reaction method, it is thought that chemical reaction can bepromoted at a nonconventional speed due to the fact that the mechanicalenergies of both sheer stress and centrifugal force are simultaneouslyapplied to the reactant and this energy is thereby converted intochemical energy.

(Metal Oxide)

Examples of a metal oxide for producing the metal oxide nanoparticlesaccording to the present invention that can be used are manganese oxideMnO, lithium iron phosphate LiFePO₄, and lithium titanate Li₄Ti₅O₁₂. Inaddition, the use of a metal oxide represented by MxOz, AxMyoz,Mx(DO4)y, and AxMy (DO4)z (wherein M: metal element and A: alkali metalor lanthanoids) as other oxides is also possible.

In case of manganese oxide MnO, for example, a composite of manganeseoxide nanoparticle precursors and carbon supporting the dispersedprecursors are produced by said mechanochemical reaction with amanganese source such as sodium permanganate, manganese acetate,manganese nitrate, and manganese sulfate together with carbon asstarting materials. By rapid heating of this composite under nitrogenatmosphere, a composite of manganese oxide, which is a metal oxide ofthe present invention and carbon is produced.

In case of lithium iron phosphate LiFePo₄, for example, a composite oflithium iron phosphate nanoparticle precursors and carbon supporting thedispersed precursors are produced by said mechanochemical reaction witha manganese source such as sodium permanganate, manganese acetate,manganese nitrate, and manganese sulfate together with carbon asstarting materials. By rapid heating of this composite under nitrogenatmosphere, a composite of lithium iron phosphate which is a metal oxideof the present invention and carbon is produced.

In case of lithium titanate Li₄Ti₅O₁₂, for example, a titanium sourcesuch as titanium alkoxide, and a lithium source such as lithium acetate,lithium nitrate, lithium carbonate, and lithium hydroxide are used asstarting materials, and the lithium titanate nanoparticle precursors areproduced by said mechanochemical reaction. The lithium titanatenanoparticles of the present invention having oxygen defect sites dopedwith nitrogen are produced by heating these lithium titanatenanoparticle precursors under nitrogen atmosphere.

(Carbon)

By adding a given carbon in the reaction process, carbon supportinghighly dispersed 5-100 nm lithium titanate can be obtained. In otherwords, a metal salt and a given reaction suppressor and carbon areintroduced into the inner tube of the reactor, and the inner tube isrotated to mix and disperse the metal salt and the reaction suppressorand the carbon. A catalyst such as sodium hydroxide is furtherintroduced while rotating the inner tube to advance hydrolysis andcondensation reaction to produce a metal oxide, and this metal oxide andcarbon are mixed in a dispersed state. Carbon supporting highlydispersed metal oxide nanoparticles can be formed by rapidly heatingthis at the end of the reaction.

The carbon employed here can include Ketjen Black, carbon black such asacetylene black, carbon nanotube, carbon nanohorn, amorphous carbon,carbon fiber, natural graphite, artificial graphite, activated carbon,mesoporous carbon, and a gas phase carbon fiber. A composite thereof canalso be employed.

(Solvent)

Alcohols, water, or a mixed solvent thereof can be employed as thesolvent. For example, a mixed solvent of acetic acid and lithium acetatedissolved in a mixture of isopropanol and water can be used.

(Reaction Suppressor)

When a metal alkoxide is used as the starting material, as described inPatent Document 2, a given compound that forms a complex with a givenmetal alkoxide can be added as the reaction suppressor, wherein saidmechanochemical reaction is applied to the metal alkoxide. This cansuppress the chemical reaction from being excessively promoted.

In other words, it was found that the reaction can be suppressed andcontrolled by adding to the metal alkoxide 1-3 moles of a given compoundsuch as acetic acid that forms a complex therewith (relative to 1 moleof the metal alkoxide) to form a complex. Composite nanoparticles of ametal and an oxide, e.g. composite nanoparticles of lithium and titaniumoxide which are lithium titanate precursors are produced by thisreaction, and lithium titanate crystals are obtained by calcinationthereof.

As stated above, it is thought that a chemical reaction can besuppressed from being excessively promoted by adding a given compoundsuch as acetic acid as the reaction suppressor because a given compoundsuch as acetic acid forms a stable complex with a metal alkoxide.

A material that can form a complex with a metal alkoxide includes acomplexing agent represented by a carboxylic acid such as acetic acid,as well as citric acid, oxalic acid, formic acid, lactic acid, tartaricacid, fumaric acid, succinic acid, propionic acid, and levulinic acid,an amino polycarboxylic acid such as EDTA, and an aminoalcohol such astriethanolamine.

(Heating)

The present invention yields a composite that supports metal oxidenanoparticle precursors inside the carbon nanoparticle structure by amechanochemical reaction, allows promotion of crystallization of metaloxide by heating this composite of metal oxide and carbon under nitrogenatmosphere, and improves the capacitance and output property of anelectrode or an electrochemical element that uses this composite.

In other words, it was found that aggregation of metal oxidenanoparticles can be prevented and nanoparticles with small particlesize are formed by rapidly heating from room temperature to 400-1000° C.in the calcination step of the obtained composite of metal oxidenanoparticle precursors and carbon nanoparticles. Rapid heating israpidly heating under a low oxygen concentration atmosphere of about1000 ppm to an extent that so that carbon is not oxidized. For example,rapid heating can be performed by for example introducing a small amountof a composite of metal oxide nanoparticle precursors and carbonnanoparticles into a calcination furnace set to the calcinationtemperature. The preferred temperature range of the heating temperaturewill differ depending on the type of metal oxide. In other words,crystallization of metal oxide proceeds well at said temperature range,wherein good crystallization progress cannot be attained at below thistemperature, and metal oxide having good energy storage property cannotbe obtained due to phase transition at over this temperature.

(Electrode)

The composite of metal oxide nanoparticles and carbon obtained by thepresent invention can be mixed and kneaded with a binder, molded, andmade into the electrode of an electrochemical element, i.e. electricenergy storage electrode. This electrode shows high output property andhigh capacitance property.

(Electrochemical Element)

An electrochemical element that can employ this electrode is anelectrochemical capacitor or battery that employs an electrolyticsolution containing metal ions such as lithium or magnesium. In otherwords, the electrode of the present invention can occlude and detachmetal ions, and works as a negative electrode or positive electrode.Accordingly, an electrochemical capacitor or battery can be configuredby employing an electrolytic solution containing metal ions, andemploying as the counter electrode for example activated carbon as wellas carbon or metal oxide which occludes and detaches metal ions.

EXAMPLES

The present invention will now be further specifically described byExamples.

Example 1

A mixed solution of 1 mole of manganese acetate, ethanol, and water wasprepared. This mixed solution and Ketjen Black (KB) were introduced intoa rotary reactor, the inner tube was rotated at a centrifugal force of66,000 N (kgms⁻²) for 5 minutes to form a thin film of the reactant onthe inner wall of the outer tube, and sheer stress and centrifugal forcewere applied to the reactant to allow promotion of a chemical reaction,yielding KB supporting highly dispersed manganese oxide precursor.

By drying the obtained KB supporting highly dispersed manganese oxideprecursors under vacuum at 80° C. for 17 hours, a composite powder inwhich highly dispersed manganese oxide precursors are supported on KBwas obtained.

By rapidly heating the obtained composite powder in which highlydispersed manganese oxide precursors are supported on KB under nitrogenatmosphere to 700° C., crystallization of manganese oxide was allowed toprogress, within 3 minutes of sintering as shown in FIG. 1, and acomposite powder in which highly dispersed manganese oxide nanoparticlesare supported on KB was obtained.

The result of XRD analysis and the TEM images of this composite powderof Example 1 are shown in FIGS. 1 to 3. It is seen from the XRD analysisshown in FIG. 1 that manganese oxide supported on KB is obtained.

As seen from the TEM images of FIGS. 1 to 3, carbon nanoparticles(Ketjen Black nanoparticles) indicate the building structure forgraphite fragment, and in particular, it is seen from FIG. 2 thatmanganese oxide nanoparticles with a small diameter (few nm) areinternalized in the carbon nanoparticles. In addition, according to FIG.3, it can be observed that graphene (KB-Graphene) which is a thin filmwherein graphite of Ketjen Black is stripped is formed, and manganeseoxide nanoparticles have entered in between the graphene in a sandwichedstate.

Subsequently, the composite powder of Example 1 configured as above wasintroduced into a SUS mesh welded onto a SUS plate together withpolyvinylidene fluoride PVDF as the binder (MnO/K13/PVDF 40:40:20). Thiswas set as the working electrode W.E. A separator as well as the counterelectrode C.E. and Li foil as the reference electrode were placed onsaid electrode, and 1.0 M lithium hexafluorophosphate (LiPF₆)/ethylenecarbonate EC:dimethyl carbonate DEC (1:1 w/w) was impregnated as theelectrolytic solution to yield a cell. In this state, with workingvoltage at 0-2 V, the energy density was calculated from its charge anddischarge property. The result showed a high energy density of 691 mAh/g(1 C) and 418 mAh/g (3 C) per manganese oxide.

Example 2-1

An aqueous solution of 1.0 mole of phosphoric acid and 1 mole of lithiumacetate relative to 1 mole of iron acetate was prepared. Citric acid wasemployed here as the reaction suppressor. This solution and carbonnanofiber (CNF) were introduced into a rotary reactor, tube was rotatedat a centrifugal force of (kgms⁻²) for 5 minutes to form a thin film ofthe reactant on the inner wall of the outer tube, and sheer stress andcentrifugal force were applied to the reactant to allow promotion ofchemical reaction, yielding CNF supporting highly dispersed olivine-typelithium iron phosphate precursor. In this case, the amounts of ironacetate, phosphoric acid, lithium acetate, and CNF to be dissolved inthe mixed solvent were set so that the composition of the compositeobtained was lithium iron phosphate/CNF at a mass ratio (w/w) of 50/50.

By drying the obtained CNF supporting highly dispersed lithium ironphosphate precursors under vacuum at 80° C. for 17 hours, a compositepowder in which highly dispersed lithium iron phosphate precursors aresupported on CNF was obtained.

By rapidly heating the obtained composite powder in which highlydispersed lithium iron phosphate precursors are supported on CNF undernitrogen atmosphere to 700° C., for a time period of 5 minutes,crystallization of lithium iron phosphate was allowed to progress,yielding a composite powder in which highly dispersed lithium ironphosphate nanoparticles are supported on CNF, as shown in FIGS. 4 to 6.

The result of XRD analysis and the TEM images of this composite powderof Example 2-1 are shown in FIGS. 4 to 6, and the charge and dischargebehavior and the capacitance calculated from this result are shown inFIGS. 8 and 9. It is seen from the XRD analysis shown in FIG. 4 thatlithium iron phosphate supported on CNF is obtained.

As seen from TME images of FIGS. 4 to 6, it can be observed that CNFnanoparticles show a structure entwined in a net. In addition, the highresolution TEM image is shown in FIG. 7. FIG. 24 is a further magnifiedphotograph of FIG. 7 and a schematic diagram thereof, and as seen fromthis FIG. 24, lithium iron phosphate nanoparticles are contained insidea snow pea-like CNF. As seen from the figure, the crystal structureappears transparent, and it is supported on a lithium iron phosphatecrystal structure (ultra thin film structure) having a thickness of 1 nmor less at 2-5 atomic layers level on a plate of 5-100 nm in diameter.

FIG. 8 is a graph showing the charge and discharge property of theelectrochemical element that uses the composite of Example 2-1. In otherwords, the composite powder of Example 1 configured as above wasintroduced into a SUS mesh welded onto a SUS plate together withpolyvinylidene fluoride PVDF as the binder (LiFePO₄/CNF/PVDF 40:40:20).This was set as the working electrode W.E. A separator as well as thecounter electrode C. E. and Li foil as the reference electrode wereplaced on said electrode, and 1.0 M lithium hexafluorophosphate(LiPF₆)/ethylene carbonate (EC):dimethyl carbonate (DEC) (1:1 w/w) wasimpregnated as the electrolytic solution to yield a cell. In this state,with working voltage at 2.0-4.2 V, its charge and discharge property wasinvestigated.

As seen from this FIG. 8, a superior capacitance property of capacitanceat 81 mAh/g per composite powder was confirmed. In addition, as shown inFIG. 9, a more superior output property was shown compared to aconventional product. In other words, FIG. 9 is a graph comparing thecapacitance per lithium iron phosphate at 60 C of the electrochemicalelement that uses this lithium iron phosphate with the approximatecapacitance of each previously reported technology. The dischargecapacitance of the element that uses the composite of the presentExample is increased compared to S. B Lee (2008), D. Kim (2006), Y. Wang(2008), and B. Kang (2009). In addition, FIG. 10 shows the outputproperty and FIG. 11 shows the cycle property, and both output propertyand cycle property are good. The discharge output property of FIG. 10was determined by measuring the discharge capacitance under a conditionsimilar to the above FIG. 8 by varying the discharge rate to1/120/180/240/300/360 C relative to charge rate 1 C. As seen from thisFIG. 10, the discharge capacitance at 360 C shows high values of 70mAh/g per lithium iron phosphate active material and 35 mAh/g percomposite. The cycle property of FIG. 11 was capable of being maintainedat 89% discharge capacitance even at 3000 cycles (10 C).

The micropore distribution was measured for the composite powder of thepresent Example by the BJH method (Barrett-Joyner-Halenda method). Asshown in FIG. 21, the micropore distribution of CNF is 10-50 nm, whilethe micropore distribution of the composite of the present applicationwas 20 nm, and it is seen that lithium iron phosphate nanoparticles aresupported in the 50 nm voids of CNF and a composite having a 20 nmmicropore distribution is formed. In other words, the microporedistribution of the composite of the present Example and CNF werecalculated, and these mesopores were observed. In the graphs of FIG. 21,the squares are the plots for the composite and the circles are for CNF.First, it is seen that CNF has more 10-50 nm mesopores than the dV/d(log r) value. In addition, a large change in micropore distribution isseen when iron phosphate is composited with this CNF. Micropore size of10-50 nm is drastically reduced, and micropore distribution at 35 around20 nm is maintained. This trend is also markedly seen with dV/dr. Fromthis result, it is speculated that the supporting of iron phosphate onCNF occurs at CNF gaps of micropore size 10 50 nm, and a network ofmesopores having micropore size of around 20 nm is further constructed.Accordingly, good ion path has been constructed in this compositeelectrode.

Example 2-2

A cell was prepared as in Example 2-1 except that lithium ironphosphate/CNF was set to be at a mass ratio (w/w) of 60/40. Thecapacitance of this cell was 71 mAh/g. In addition, the high resolutionTEM image of this composite powder is shown in FIG. 12. As seen fromthis figure, a lithium iron phosphate crystal structure having athickness of 1 nm or less at 2-5 atomic layers level on a plate of 5-100nm in diameter is supported on CNF.

Example 2-3

A cell was prepared as in Example 2-1 except that Ketjen Black wasemployed as the carbon. The capacitance of this cell was 108 mAh/g. Inaddition, the high resolution TEM image of this composite powder isshown in FIG. 13. As seen from this figure, a lithium iron phosphatecrystal structure having a thickness of 1 nm or less at 2-5 atomiclayers level on a plate of 5-20 nm in diameter is internalized in KetjenBlack. FIG. 25 shows the high resolution TEM image of this compositepowder of Example 2-3 and a schematic diagram thereof. This Example 2-3has a structure wherein one lithium iron phosphate nanoparticle isplaced inside each lantern plant-like hollow spherical carbon.

Example 2-4

A cell was prepared as in Example 2-3 except that lithium ironphosphate/Ketjen Black was set to be at a mass ratio (w/w) of 60/40. Thecapacitance of this cell was 102 mAh/g.

Example 2-5

A cell was prepared as in Example 2-1 except that BP2000 available fromCabot Corporation was employed as the carbon. The capacitance of thiscell was 88 mAh/g. In addition, the high resolution TEM image of thiscomposite powder is shown in FIG. 14. As seen from this Figure, alithium iron phosphate crystal structure having a thickness of 1 nm orless at 2-5 atomic layers level on a plate of 5-100 nm in diameter issupported on BP2000.

Example 2-6

A cell was prepared as in Example 2-3 except that lithium ironphosphate/BP2000 was set to be at a mass ratio (w/w) of 60/40. Thecapacitance of this cell was 96 mAh/g.

Example 3

Acetic acid and lithium acetate in amounts of 1.8 moles of acetic acidand 1 mole of lithium acetate relative to 1 mole of titanium alkoxidewere dissolved in a mixture of isopropanol and water to prepare a mixedsolvent. This mixed solvent together with titanium alkoxide and carbonnanofiber (CNF) were introduced into a rotary reactor, the inner tubewas rotated at a centrifugal force of 66,000 N (kgms⁻²) for 5 minutes toform a thin film of the reactant on the inner wall of the outer tube,and sheer stress and centrifugal force were applied to the reactant toallow promotion of chemical reaction, yielding CNF supporting highlydispersed lithium titanate precursor. In this case, the amounts oftitanium alkoxide and CNF dissolved in the mixed solvent were set sothat the composition obtained was lithium titanate/CNF at of 70/30.

By drying the obtained CNF supporting highly dispersed lithium titanateprecursors under vacuum at 80° C. for 17 hours, a composite powder undervacuum at in which highly dispersed lithium titanate precursors aresupported on 10 CNF was obtained.

The obtained composite powder in which highly dispersed lithium titanateprecursors are supported on CNF was rapidly heated under nitrogenatmosphere to 800° C. to allow progression of crystallization oftitanium oxide containing lithium, and a composite powder in whichhighly dispersed lithium titanate nanoparticles are supported on CNF wasobtained.

The TEM image of the carbon supporting lithium titanate nanoparticles ofExample 3 obtained as above is shown in FIG. 15. In FIG. 15, it is seenthat highly dispersed 5 nm-20 nm lithium titanate nanoparticles aresupported on CNF.

In particular, as seen in the TEM image of FIG. 15, “the composite oflithium titanate nanoparticles and carbon” of the present inventiontakes a form of a “building structure for graphite fragment” of CNFconnected together, and highly dispersed lithium titanate nanoparticlesare supported on this structure.

FIG. 16 shows a figure of the CNF supporting highly dispersed lithiumtitanate precursors of Example 3 observed with a high resolution TEM. Asseen from FIG. 16, the crystal structure of the lithium titanatenanoparticles appears transparent, and is a lithium titanate crystalstructure having a thickness of 1 nm or less at 2-5 atomic layers levelon a plate with a 5-10 nm side. Such ultra thin film structure hasextremely thin thickness, extremely large property. Accordingly, it canshow a high output property.

In other words, in regards to the surface area per volume, the surfacearea of a sheet having a thickness infinitely close to zero is thelargest, and the sheet of Example 3 has a structure having a thicknessof a few atomic layers level close to zero. The above ultra thin filmstructure is thought to be formed by applying sheer stress andcentrifugal force to a solution comprising the starting material ofmetal oxide and carbon powder in a rotating reactor to allow reaction,and then subjecting to rapid heating treatment, but as observed withlithium iron phosphate, metal oxide nanoparticles other than lithiumtitanate also has an ultra thin film structure.

The composite powder obtained in Example 3 configured as above wasintroduced into a SUS mesh welded onto a SUS plate together withpolyvinylidene fluoride PVDF as the binder (Li₄Ti₅O₁₂/CNF/PVDF56:24:20). This was set as the working electrode W. E. A separator aswell as the counter electrode C. E. and Li foil as the referenceelectrode were placed on said electrode, and 1.0 M lithiumtetrafluoroborate (LiBF₄)/ethylene (EC):dimethyl carbonate (DEC) (1:1w/w) was impregnated as the electrolytic solution to yield a cell.

For cells having an electrode that employs the composite powder ofExample 3 obtained as above and Comparative Example 1 heated under thesame condition under vacuum, the charge and discharge behavior thereofand capacitance calculated based thereon are shown in FIG. 17, and theoutput property is shown in FIG. 18. In FIGS. 17 and 18, the left graphshows Example 3, and the right graph shows Comparative Example 1. Inthis case, the working voltage is 1.0-3.0 V and the scan rate is 10 C.

As seen from FIG. 17, a cell that uses the composite powder of Example 3heated under nitrogen atmosphere has increased capacitance compared to acell that uses the composite powder of Comparative Example 1 heatedunder vacuum. In particular, a cell that uses the composite powder ofComparative Example 1 heated under vacuum to 800° C. for 3 minutes hadthe largest capacitance among conventional technology, but the cells ofExample 3 all had capacitance far greater than that of Comparative 15Example 1, see heated under nitrogen gas to 800° C. for 3 minutes inFIG. 17.

FIG. 18 is graphs showing the output property of each cell with C-rateon the horizontal axis and discharge capacitance maintenance rate (%) onthe 20 vertical axis. As seen from this FIG. 18, the dischargecapacitance maintenance rate when the C-rate is at 200 C is far greaterfor the cell of Example 3 than the cell of Comparative Example 1.

Example 4

An electrochemical element was prepared by employing the workingelectrode prepared in Example 2-1 as the positive electrode, the workingelectrode prepared in Example 3 as the negative electrode, and 1.0 Mlithium hexafluorophosphate (LiPF₆/ethylene carbonate (EC):dimethylcarbonate (DMC) (1:1 w/w) as the electrolytic solution. The result ofmeasuring the energy density and power density for this electrochemicalelement is shown in FIG. 19.

This FIG. 19 is a Ragone Plot of measuring the energy density and theoutput property for each of the electrochemical element of Example 4, anelectrochemical element that uses activated carbon electrode as thepositive electrode and the working electrode produced in Example 3 asthe negative electrode, and an electric double layer capacitor (EDLC)that uses activated carbon for the positive electrode and the negativeelectrode. As seen from this FIG. 19, the electrochemical element ofExample 4 realizes a high output energy storage device having highenergy density and high output property.

Example 5

For the synthesis of the Li₄Ti₅O₁₂/CNF composite, Ti(OC₄H₉)₄ wasemployed as the titanium source and CH₃COOLi as the lithium source.These raw materials were subjected to ultracentrifugation treatment (UCtreatment) together with 10-40 wt % of CNF relative to totalLi₄Ti₅O₁₂/CNF and an organic solvent, etc. to yield a precursor. Highcrystalline Li₄Ti₅O₁₂/CNF composite nanoparticles were then obtained byhigh-temperature short-duration calcination. Electrochemical propertywas evaluated by a half cell that employs this composite made into anelectrode by employing PVDF, Li metal as the counter electrode, and 1 MLiBF₄/EC+DMC 1:1 (in volume) as the electrolytic solution. As a resultof charge and discharge test, the output property was found to bedependent on the weight ratio on Li₄Ti₅O₁₂. In addition, as seen fromFIG. 22, 81% (87 mAh/g) of the 10 C capacitance at 600 C that demandshigh output property was maintained, and further 68% (72 mAh/g)capacitance at 1200 C was maintained.

As a result of XRD analysis of this composite of Example 5, as shown inFIG. 23, the crystal size of (111) face with CNF content ratio of 20%was larger than that of 30-50%, and it was confirmed that the lithiumtitanate nanoparticle crystal has an ultra thin film structure withlarge (111) face.

What is claimed is:
 1. A composite of metal oxide nanoparticles andcarbon, wherein each of the metal oxide nanoparticles is placed insideKetjen Black of a separate hollow spherical carbon structure having anexterior rough surface configuration around the metal oxidenanoparticles, the mass ratio (w/w) of the metal oxide nanoparticles tothe Ketjen Black is set at 50/50 to 60/40.
 2. The composite of metaloxide nanoparticles and carbon according to claim 1 where a lithium ironphosphate crystal structure having a thickness of 1 nm or less and a 2-5atomic layer level is internalized in the Ketjen Black of the hollowspherical carbon structure.
 3. The composite of metal oxidenanoparticles and carbon according to claim 2 wherein the metal oxidenanoparticles are lithium iron phosphates.
 4. An electrode that employsthe composite of metal oxide nanoparticles and carbon according toclaim
 1. 5. An electrochemical element that employs the electrodeaccording to claim
 4. 6. The electrochemical element according to claim5 that employs lithium iron phosphate LiFePO₄ as a positive electrodeactive material and lithium titanate Li₄Ti₅O₁₂ as a negative electrodeactive material.
 7. A cell that employs the composite, of metal oxidenanoparticles and carbon according to claim 1 where said cell is formedwith a capacitance of 102 mAh/g.